106 University of California Publications in Zoology. L^^'ol. 4 



inequalities in illumination due to different objects, will the star- 

 fish move toward the shade cast by its own body, or toward that 

 due to some other object" ? 



Experiments answer this question in exactly the way that 

 might be anticipated, when it is put in this form. The starfish 

 sometimes moves toward the shadow cast by its own body {i. e.. 

 in the direction of the rays of light), sometimes toward the 

 shadow cast by some other body {i. e., toward the less illumin- 

 ated part of the field). Which of the two factors shall prevail 

 when both are present appears to depend, other things being 

 equal, on the relative extent of the two shaded regions. It is 

 not possible to compare precisely the extent of shade cast by 

 the body of the starfish with that due to some other object, so 

 that the relation cannot be expressed in an accurate quantitative 

 way. ]\roreover, the shade cast by the starfish itself connnonly 

 affects mainly the ventral surface of the animal, while that due 

 to other objects affects mainly the dorsal surface, so that the 

 effects of the two may not be strictly comparable. But the fol- 

 lowing experimental results show that the general relation is as 

 stated above. 



(1) If the starfish is placed in a flat vessel with the sun 

 shining on it from one side, and no part of its body is screened 

 from the sun, it of course moves directly away from the sun 

 (i. €., toward the shadow cast by its body). But if that half 

 of its body next to the sun is shaded by a black screen, so that 

 only the distal half is in the light, then the starfish moves as a 

 rule into the shadow cast by the screen, and thus toward the 

 sun and against the sun's rays. If less than one-half the body 

 was shaded in this way, the results were variable. Sometimes 

 the starfish moved into the shade of the screen (and toward 

 the sun) when only a part of certain rays was shaded; at other 

 times it moved under such conditions away from the source of 

 light. If the experiments are carried on in a flat vessel with 

 opaque sides, the sunlight coming slantwise from above, the star- 

 fish if partly shaded by the side of the vessel next the sun often 

 moves toward that side, hiding itself in the shadow. 



° It would appear that the problem might be similarly put for any other 

 organism. 



